Snooker in time

Alan Robinson asks if there are any time-travel paradoxes that do not involve sentient beings (11 October, p 30). I can recall one from an old issue of this magazine.

Imagine a snooker table on which the corner pocket is the mouth of a wormhole that leads to a position on the table 1 second in the past. For certain trajectories, the ball emerging from the wormhole will deflect its earlier self and thus prevent it from entering the hole in the first place.

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